The 100K-dalton human tumor-associated antigen, originally detected in spend culture medium of human melanoma cells, has been characterized biochemically and functionally, and has been preliminarily assessed for utility and immunoprognosis of human melanoma. The rationale for these studies are that this glycoprotein, defined by monoclonal antibody, represented a different class of melanoma antigen than previously described; i.e., it is predominantly secreted rather than expressed at the cell surface and thus offers potential as a serum marker. Serum levels were shown to correlate with tumor burden in melanoma patients. The 100K antigen was shown to also be present in normal serum, at levels of 100 ng/ml or less, complexed in a non-covalent manner with human serum albumin. The presence of normal serum was used to determine the phylogenetic distribution of the tumor marker: the antigen was present in sera and in the spent culture medium of cells from humans and higher apes but not New World monkeys or lower animals, indicating the antigen is a rather late evolutionary development. Preliminary data indicate the 100K MAA may share similarities with C-reactive protein, a primitive immunoglobulin-like acute phase reactant. Studies are continuing to determine its usefulness in diagnosis and prognosis, and possibly its functinal role, e.g., its influence on host immune responses.